Atomic Coherence and Optical Storage

Index

Foreword

In quantum mechanics it is known that if a transition takes place from an initial state to a final state via several different paths then one should add coherently all transition amplitudes before calculating the expression for transition probability. The coherent addition leads to quantum interferences and these are quite ubiquitous for quantum systems. Further any interactions in the final state would significantly affect the transitions rates. This was first extensively discussed by Fano and led to flurry of activity both theoretical and experimental. Such interferences are even more pronounced for higher order processes. Early experiments on multi photon processes and more specifically on the competition of one photon and three photon processes showed very remarkable role of interferences. The coherent path ways naturally lead to the production of atomic coherences. About two decades ago the scientific community realized the great potential ofquantum interferences largely due to the work of Steve Harris and collaborators. By now we have seen many important applications of quantum interferences and atomic coherences and many ofthese applications have been thoroughly reviewed. However newer and newer applications continue to appear and one needs a good discussion at one place rather than going through the large volume of papers.

The current book by Gao and collaborators fills a gap and provides a comprehensive discussion ofa variety of important new applications. These for example include- methods to reach the white cavity limit; production of nano scale resolution for microscopy and lithography applications; production of controllable photonic band gaps; creation of memory elements.

The material has been prepared by authors who themselves have extensive contributions to the field. The book is timely and should be quite useful for young researchers and those who like to get a feel for the great potential of quantum interferences and atomic coherences.

Preface

Laser-induced atomic coherence and interference, and related effects in multi-level atomic and molecular systems have been actively studied in 70's and 80's as important techniques for laser spectroscopy. This field was reenergized after the first experimental demonstration of electromagnetically induced transparency (EIT) in three-level atomic gas systems about twenty years ago by the group of Professor S.E. Harris at Stanford University. Such EIT effects were later observed in atomic vapor cells with low-power diode lasers under two-photon Doppler-free configurations and in cold atoms confined in magneto-optical traps, as well as in many molecular and solid material systems. In the past twenty years, many interesting phenomena related to laser-induced atomic coherence and interference were experimentally demonstrated, such as slow and superluminal light propagations, lasing without inversion, enhanced refractive index, enhanced nonlinear optical processes, matched light pulses, optical memory, correlated/entangled photon pairs, controllable optical bistability, and cavity-QED effects. The atomic coherence effects in multi-level media have been shown to have potential applications in all-optical switching/router, logic gates, precision measurements, optical buffer/delay lines for optical communication and computing, and quantum information processing.

There have been several well-written review articles published in recent years to cover various aspects of atomic coherence and interference in multi-level systems. In this e-book, we put together reviews of several research topics related to laser-induced atomic coherence and interference, and its potential applications in atomic and solid media written by active researchers working in these fields. We hope that this e-book can serve as a good reference for graduate students and researchers interested in acquiring some general understanding and perspective of this active research field.

Chapter 1 presents experimental studies of EIT-enhanced linear and nonlinear dispersions of three-level atomic medium inside an optical cavity. By balancing the sharp linear and Kerr-nonlinear dispersions of the atomic system near the EIT resonance, the total intracavity atomic dispersion can be made to be either normal or anomalous, which leads to subluminal (slow) light propagation or superluminal (fast) light propagation in the cavity, and concomitantly, results in a narrowed or broadened cavity linewidth compared to the empty cavity case. Under certain experimental conditions, the so called "white-light cavity" can be realized with a very broad transmission spectrum, which can have applications in nonlinear precision spectroscopy and recycling cavity of the laser interferometer for the gravitational-wave detection. Chapter 2 shows that by controlling the phases between various laser beams interacting with multi-level atomic systems, phase-dependent quantum interference is induced in the atomic systems and either constructive or destructive interference can be obtained in the probe transitions. For example, when bichromatic coupling and probe fields are used in a three-level system, the interference between the resonant two-photon Raman transitions can be controlled by varying the relative phases of the coupling or probe fields. Also, when two coupling fields and two probe fields are used in a four-level double-Λ system, manipulating the relative phase among the laser beams creates interference between three-photon and one-photon excitation processes, which can be used to selectively enhance or suppress probe light absorptions. Both experimental studies and simple theoretical descriptions are presented to illustrate such phase-dependent atomic coherence effects.

In Chapter 3, several schemes for realizing atomic localization via atomic coherence and quantum interference in multi-level atomic systems are proposed, including double-dark resonance effects, subhalf-wavelength localization via two standing-wave fields, and two-dimensional localization by using two orthogonal standing-wave fields. Using double-dark resonances, the detecting probability and localization precision for the atoms can be greatly enhanced. Also, using standing-wave fields in ladder-type system can improve the detecting probability and lead to sub-half-wavelength localization precision in two-dimension, which provides potential applications in two-dimensional nano-lithograph.

Chapter 4 presents some recent theoretical studies on quantum correlation and entanglement properties of the four-wave mixing and quantum-beat laser systems in multi-level atomic systems. The four-wave mixing can yield Einstein-Podolsky-Rosen (EPR) entangled states by simultaneously absorbing in the excitations from a pair of squeeze-transformed modes, and the quantum-beat laserscan act as bright sources of entangled light beams with sub-Poissonian photon statistics.

Chapter 5 describes few techniques to generate and tune the photonic bandgaps of the gratings written
by standing-wave fields in coherent media such as cold rubidium atoms, Pr3+: Y2SiO4, and diamond
containing N-V color centers. Both steady and dynamic optical responses of the media exhibiting
induced photonic bandgaps have been theoretically examined in terms of transmitted and reflected
spectra or pulse propagation dynamics. The standing-wave driving configuration is shown to be an
efficient technique to control light flows and optical nonlinearities with spatially periodic quantum
coherence, which may be exploited to achieve all optical switching, router, and storage and thus has
potential applications in quantum information processing. Chapter 6 shows several experimental
demonstrations of light storage based on atomic coherence and atomic coherence transfer. By
preparing maximal atomic coherence in the coherently-driven media, light storage based on F-STIRAP
is realized in atomic vapor and solid-state crystal respectively. Some applications of EIT-based
light storage in solid Pr3++:Y2SiO5 crystal are studied experimentally. Also, by employing STIRAP process, atomic coherence transfer between different spin levels is proposed theoretically and
demonstrated experimentally.

As one can see that this e-book has covered a broad spectrum of research topics from experimental demonstrations of EIT-related effects (such as cavity linewidth modification, phase-dependent interference, and optical storage) to several interesting theoretical predictions (e.g. localization of atoms, quantum correlation, and tunable photonic bandgaps). Also, in the experimental demonstrations of atomic coherence effects, different multi-level media, such as hot atomic cell in an optical cavity (Chapter 1), cold atoms in the magneto-optical trap (Chapter 2), and solid crystal at low temperature (Chapter 6), are used, which show the broad applicability of atomic coherence effects and their potential applications.